Sound absorbing material and method of making the same



W. T. DEAN Aug. 2, 1932.

SOUND ABSORBING MATERIAL AND METHOD OF MAKING THE SAME Filed Dec. 20, 1930 Patented Aug. 2, 1932 UNITED STATES PATENT OFFICE I WILLIAM '1. DEAN, OF GARY, INDIANA, ASSIGNOR TO CALICEL PRODUCTS, INC., 01:

HAMMOND, INDIANA, A CORPORATION OF ILLINOIS SOUND ABSORIBING MATERIAL AND ME'I HOD OF MAKING THE SAME Application med December 20, 1930. Serial No. 503,680.

This invention has to do with sound absorbing materials for the correction of resonant effects in halls, theaters and other auditoriums, offices and other places where such correction is desirable, as well as for deadening sounds in any and all other situations where resonance is objectionable, such as in factories, machine shops, etc. It includes not only an improved sound absorbing material, but also the process or method by which it is produced and made available for commercial use.

. The primary object of the invention is to provide a material which may be conveniently applied to the walls and ceiling surfaces of chambers, and which will as nearly as possible absorb all sound waves impinging thereon, and at the same time will be fireproof, thereby avoiding the fire hazard incident to the use of inflammable or combustible sound proofing materials, such as have heretofore been used.

Another object of the present invention is to provide a sound absorbin material which, together with suitable backing and or reenfo rcing members, can be formed or cast as relatively large units which may be used in lieu of lath and plaster, or be placed against and secured to wall or ceiling surfaces, or any other surface to be protected with the sound absorbing surface exposed and left open and porous. In the accompanying drawing which illustrates how the latter object may well be accomplished,

Figure l is a sectional view illustrating a portion of a unit formed with a backing of plaster, cement or wallboard with the-sound absorbing material superimposed thereon;

Figure 2 is a sectional view similar to Figure 1, but illustrating the unit with reenforcing wire mesh incorporated therein; and also provided with means for attaching the unit to a building structure;

Figure 3 is a sectional view illustrating another way of attaching the units to a builds ing structure; and

Figure 4 is a sectional view illustrating still another way of attaching the units to a building structure, and also illustrating means for sealing the. joints between adjacent units.

It is old in the art to employ sound absorbing materials composed principally of vegetable fibers loosely matted together in the form of a comparatively thick board having its surface indented or scored to allow access of the sound waves to the interior of the fibrous mass, but material of that kind has a relatively low coefficient of sound absorption, and although it is usually treated to make it more or less fire resistant, it never becomes fireproof, and hence its use increases the fire hazard wherever it is applied. It

has also been proposed to use sound absorbing material composed principally of mineral fibers, such as mineral wool, but such fibers are subject to matting and slow decomposition into dust when used dry, and, like the material composed of vegetable fibers, have but slight sound absorbing effect when bonded into solid masses. Still another form of sound absorbing material that has been proposed is made up of rock-like material crushed or broken up into small pieces of uniform size and shape and bonded together by cementitious material so as to form a solid block or mass having interstices between adjoining particles, some of which are exposed at the surface of the mass for the purpose of permitting the sound waves to enter and be absorbed in the body of the mass. The foregoing and various other forms of sound absorbing material have been employed to a greater or less extent, but none of them, so far as I am aware, has been very efficient for its intended purpose, for various reasons, and none of them has a sufiiciently high co-efficient of sound absorption to be generally satisfactory.

As is well understood, the sound absorbing properties of various materials are usually rated in terms of their proportion of sound absorption as compared with the complete sound absorbing property of a relatively large opening into a resonant chamher, which is taken as 100% in all comparative tests of acoustical materials. A completely sound absorbing material is rated as ing material of equivalent thickness prior to my invention is about .46. In contrast, my improved sound absorbing material hereinafter described rates by test at from .59 to .70 or better, so that it is far superior to any material of equivalent thickness for the purpose previously known.

In my pending applications, Serial No. 331,716, filed January 10, 1929, and Serial No. 575,119, filed November 14, 1931, I have described a material that may conveniently be derived from molten blast furnace slag, and which, owing to its sponge-like appearance, I have there termed rock sponge. It may be produced by melting raw rock, sand and shale in suitable furnaces and causing a stream of water at considerable pressure, usually about thirty pounds per square inch, and at normal temperature, say 60 to 70 F., to impinge on the molten stream as it issues from the furnace, but as above suggested, it may conveniently and economically be obtained by similar treatment of the molten sla flowing from blast and other metallurgica furnaces. By thus cooling the slag by substantially the maximum quantity of water into steam, the cooling of the molten silicates is conducted slowly enough to permit the occluded gases to form bubbles, but rapidly enough to cause the material to congeal and trap the small bubbles before they have time to combine into larger ones and escape. By this process the slag expands relatively slowly into great masses of highly porous rock, whereas if the slag be cooled by an eX- cess of water, as by discharging it into a tank, it will break up "iolently into small particles light n weight but heavier than water. When the cooling medium is air or steam at high pressure, the resulting product is What is commonly termed rock wool, which has a fibrous structure of a glassy character, without the cellular characteristics of what I have termed rock sponge. The latter is a prodnot of extremely low density, having myriads of cells that vary more or less in size throughout the mass and inter-communicate through passages or channels of various sizes. Examined under the microscope it appears that such cells are interconnecting in both planes, and that they vary from irregular openings as large as one-eighth inch in diameter to openings the smallest of which is discernible only under the microscope. This characteristic of rock sponge makes it peculiarly suitable for use as soundabsorbing material, since it enables the sound waves to penetrate to the interior of the mass, with gradual absorption therein, the entire body of the mass functioning to that end, instead of being limited to a comparatively few channels or spaces provided between adjoining particles of a composite mass made up of solid particles cemented together. Practical tests have shown that my improved material is not only highly absorbent at those musical pitches that correspond to the ordinary tones of the human voice, but also has the property, to an even greater extent, of absorbing the troublesome over-tones and higher pitches. This is unquestionably due to the enormous area of microscopically small free canals for the passage of sound waves into and throughout the cellular mass. It is evident that the efficacy of the material for sound absorbing purposes depends to a considerable extent upon the presence of the voids or cells throughout the mass, and would disappear or be greatly reduced if such voids were filled up with water or cementitious material, as that would prevent the penetration of the sound waves into the mass and their dissipation therein. This material, although it resembles sponge in appearance lacks the characteristic capillary structure of natural sponge, and, therefore, is non-absorbent except superficially. Consequently, as will hereinafter more fully appear, a multiplicity of small particles of the material may be successfully bonded together to form a comparatively large block without materially detracting from its sound absorbing quality, since the bonding agent does not penetrate into the body of the mass and fill its cells and passages.

In making the desired product from rock sponge, or equivalent expanded molten siliceous material, such as may be produced by melting limestone, sandstone, and/0r aluminiferrous clays and sl1ales,thecrude rock sponge, which usually comes in comparatively large irregular masses, is reduced to comparatively small particles of irregular size and shape, without crushing it so as to form any considerable proportion of fines. To avoid such crushing, subdivision may be accomplished by slicing the crude material, passing it through cracking rolls, or forcing it through small but sharp edged screen apertures of suitable size to reduce the material to comparatively small particles having the desired dimensions. Though small, these particles are of appreciable mass and are in the form of granules having the characteristic multi-cellular and passage structure, and in size range preferably from one-half. inch down.

These granules are then superficially cemented together by means of a suitable bonding agent. It is essential that the bonding agent be of such a nature that it shall not close the pores or cells within the granules to an appreciable extent, since it would be fatal to use a soluble bonding agent of such physical character that it would fill up all of the pores so as to displace the air normally contained in them and leave them filled with a more or less liquid or solid substance. I have found that a viscous bonding agent, if applied in the viscous state without undue dilution, will coat the exposed surfaces of the particles so that when they are pressed together into a form to make an integral 1 mass, successful bonding occurs without filling the pores or cells. The cell partitions of the particles of'rock sponge are relatively fragile, and when the material is pressed into a form, they tend to break down at their points of contact, thus increasing the bonding effect. The extent to which the particles are thus broken down, and the resultant area of interstices between adjoining particles is wholly dependent upon the pressure used in forcing them into the form. Hence, the acoustical properties of the block may be varied to a certain extent by varying the density of packing of the particles in the form. Thus, if the particles be left in air extremely loose state-merely stuck together lightly at their points of contact, the resulting acoustical block will havea great number offrelatively large openings between the granules, whichwill supplement the cells and passages within the body of each particle, thereby absorbing a larger percentage of the low pitch sound waves, whereas, if the particles be compressed to a greater extent, so as to eliminate the space between adjoining particles, the sound absorbing property then depends altogether upon the cellular structure of the individual particles, which, in the main, being small, tends to increase the absorption of the higher pitch waves. It will be understood however, that good absorption for all pitches is attainable by the use of blocks in which the interstices are substantially or entirely eliminated, and I prefer to use. this construction, because the higher pitch waves are more objectionable.

In view of the multi-cellular structure of the material with the inter-communicating passages between cells, it will be apparent that in whatever plane an individual parti- 1 cle happens to lie when cemented to an adjoining particle there will be a multiplicity of minute canals that extend from the surface of the block of acoustical material into the body of the mass, and that the eflicacy of such canals for the admission of sound waves will not be dependent upon, although it may be aided by, any channels or passages that may remain between adjoining particles. As the particles are irregular in shape, and of different sizes, they will tend to so arrange themselves that the smaller particles will fill the interstices between the larger ones, and less crushing and less binder will be required to produce a homogeneous block free ofinterstices. I have found that the rock sponge particles themselves are equal or superior to the interstices between the particles in pre-- vious acoustic materials.

Bonding materials suitable for uniting the cellular particles into a coherent sound absorbing block or'slab include the starches, dextrin, casein, albumen, vegetable or animal glues, waxes, gums, coal tar, asphalt derivatives, sodium silicate and like substances hav ing high viscosity or high surface tension of the liquid. I preferably employ a bonding agent that is white or light in color, such as ordinary corn starch, which has been found to be an excellent binder, being strong, light in weight, and in itself a fairly good sound absorbing material. In the cooked state, precisely as starch is prepared for laundry purposes,-it has been found that approximately seven ounces of dry starch with three pounds of concentrated sodium silicate will produce sufficient bonding material for a cubic foot of rock sponge, and that the plastic mixture, when placed in a mold and subjected to drying operations, becomes hard and firm and suitable for the purpose desired.

, Where reenforcement is desirable, as for example, when the sound absorbing material is required in relatively large units or for use in lieu of lath and plaster, a reenforcing element, such as a section of burlap, porous paper, metallic wire mesh, perforated metal, or any ordinary wall board, is placed in a form, with or without a portion of freshly mixed gypsum plaster, Portland cement or or plaster with the sound absorbing material 6 superimposed there pn, while in Figure 2, I have shown a similar unit provided with wire mesh 7 imbedded therein for further reenforcing the same.

By selecting suitable reenforcement with supplementary backing materials of plaster, cement or wallboard the units of sound absorbing materials may be made as large as architectural needs may demand so as to dispense with the usual lath and plaster of side Walls and ceilings and to serve as the entire body of a suspended ceiling wherever such construction is desirable.

Where such reenforced units of sound absorbing material are used they may be attached directly to studding or joists, whether of wood, metal, ceramic materials, gypsum or concrete, by means of wire loops 8 imbedded in the body of the unit and extending outwardly from the backing member 5 thereof, as shown in Figures 2 and 4, or screws, toggle bolts, or similar fastening devices may be used for such purposes. In Figure 3 I have illustrated another way in which the units may be secured in position as there shown, the units are provided with circumferential grooves 9 molded in the edges thereof, which grooves serve to support the units by proper registration with previously erected channelirons, T-irons or I-beams, indicated by the numeral'lO. The combined structural and sound absorbing units may be reenforced with integrally formed wires or loops 8 as shown in Figure 4, for attachment to prepared ceiliug suspension members 11, or side wall attachment members. Thus, my sound absorbing rcen forced structural units may be attached directly to steel beams, columns, or girders, or may be suspended therefrom, serving at once as fire proofing therefor and structural finish, or they may be attached to brick, tile or concrete walls or tile or concrete ceiling beams, or suspended therefrom, serving as structural units spanning the spaces between columns or beams and at the same time providing a sound absorbing, heat insulating finished interior surface for the space so treated. The economies in lath, plaster, suspended ceiling construction and labor, secured by my construction, are important and new to the arts involved.

Since the combined sound absorbing and structural units are molded, the thickness of sound absorbing material and reenforcing material may be varied to meet required conditions, and the exposed surface of the sound absorbing material may take the form of plane or curved surfaces, raised or depressed panels or ornamental designs.

Since the base of my combined sound absorbing and structural unit is of cementitious material the joints between adjoining sections may be effectually sealed with similar ccmentitious materials at the time of erection as shown at 12 in Figure 4, thus remedying the common defects found in other forms of combined sound absorbing and structural units which allow infiltration of dust laden air between adjacent units, or through noninipervious sound absorbing units, with resultant damage to decorative finish.

For sound absorbing purposes it has been found that by finishing or cutting down such exposed dry surface, as by grinding with an emery wheel or sanding wheel, the sound absorbing properties of the material are increased and the exposed surface is improved in appearance. By the grinding operation any bonding material thereon that might interfere with access of the sound waves to the cells and passages of the sound absorbing material is removed, leaving the cellular structure thereof exposed so that its intrinsic sound absorbing properties are not impaired. The above described treatment of the exposed surfaces of my material is not essential to a high degree of sound absorption but supplemental thereto.

Successful bonding withoutfilling the pores assess? of the material may also be accomplished.

by previously wetting it and then dusting over the wet particles a dry bonding agent which absorbs its moisture from the partlcles themselves. For this purpose Portland cement, gypsum plaster, dry sodium silicate, dry powdered albumen, and similar substances may be used. Where organic bonding agents are employed it is desirable and important that some substancdbe added that will positively prevent the persistence of bacteria or other life in the block, which would lead to disintegration, discoloration, or other forms of impermanence. Suitable substances for this purpose are simple alkalis such as caustic soda, ammonia, or neu tral salts such as sodium fluoride, or organic germicides such as para-formaldehyde.

In cases where it is necessary to increase the strength of the finished product a small amount of fiber may be added to the plastic mixture without detracting from its sound absorbing properties, such as curled hair, sisal fiber, cotton fiber, or any other of the vegetable fibers. Owing to the small amount of fiber required, the fireproof quality of the block is not affected.

It will be understood, of course, that the finished sound absorbing product may be of any size or shape suitable for the use to which it is to be put. It may be preformed into tile, mouldings, and other shaped designs as desired by architects or artists, and may be colored in whole or in part for any desired decorative effect; or the loose particles of material maybe combined With asuitable binder and applied directly to the walls, ceiling or other surface requiring treatment, as a hand trowled plaster, or be deposited upon such surface by means of a stream of air. It is so light in weight that sections or panels intended for acoustical purposes may readily be attached to any plain surface without the use of nails, screws, furring strips or pressed metal forms of any kind, by simply smearing a small amount of freshly mixed gypsum plaster, or any good glue, on the back of the section and then holding it firmly for a few moments against the surface to be treated. Thus acoustical protectionmay be made correctly and cheaply with a minimum of disturbance to existing walls or equipment. The exposed face ofthe material may be decorated with Water color or lacquers before applying it to the wall or other surface to which it is to be affixed, or the bonding material may be colored with dyes to secure the desired color effect. By tinting the exposed face of the sound absorbing material prior to the grinding above referred to, under and over-tones of color are readily obtainable, and the decorative effects that may be achieved are almost unlimited. The application of a lacquer to the exposed face of the material has a tendency to strengthen it, and so long as the lacquer or other decorative coating applied to it is not permitted to choke up the cells and displace the air normally present in them, the sound absorbing property of the material will not be materially impaired, since, even though some of the outer cells should be filled to a greateror less extent, the main body of underlying cells will remain open for the receptionof sound waves and their absorption within the body of the material.

My improved sound absorbing material may also be employed in a loose granular form by confining the granules between membranes, such as cloth, wire screen, perforated metal, etc., suitable to hold the granules in close association with one another in the form of a slab or block, without preventing access to them of the sound waves. As the sound absorbing property of the material does not depend upon the presence of intersticial canals, it may be formed into tile or other commercial shapes by subjecting a mass of the particles to hydraulic or other suitable pressure, provided, of course, the pressure is not great enough to crush the particles sutficiently to destroy their cellular character. Where such pressure is used only an extremely small amount of bonding agent is required to produce tile having the requisite mechanical strength while still retaining a coeificient of absorption approximating 7 5% of total or perfect absorption.

It is obvious that my sound absorbing material as herein described may be applied as a single or multiple plaster to existing walls and ceilings or may be preformed as a part of or attached to pre-cast roofing or wall tile or slabs of any sort so that when such slabs are placed in the structure of a building, the sound absorbing surfacemay become the exposed finished surface within the space wherein acoustical correction is desired.

It will be seen from what has been said that my improved sound absorbing material consists of an assembled mass of comparatively minute particles of expanded molten silicates, termed rock sponge, the individual particles being of irregular sizes and shapes, and characterized by a countless number of cells intercommunicating by passages through which sound waves m'ay permeate the body of the particle until absorbed, such particles being held in close contact with one another, as by a bonding agent of such nature that it will not displace the air from the pores and passages and close them so as to prevent access of sound waves thereto. So far as I am aware, a material of this character is generically new, and the claims hereinafter made are therefore to be construed, accordingly.

I claim:

1. The method of forming an acoustic material which comprises mixing small particles of rock sponge with a suitable bonding agent, compressing the mixture of particles with a sufiicient degree of pressure to at least partially break down the contacting edges of the particles and thereby eliminate the interstices between the particles to a corresponding extent, and then hardening the bonding agent without changing the relative position of the particles.

2. The method of making an insulating material which comprises taking small particles of rock sponge, mixing said particles with a suitable bonding agent, subjecting the mixture to high pressure, hardening the bondlng'agent without changing the relative posit1on of the particles and trimming that part of the surface of the resulting block which is intended to be exposed.

3. The method of making an acoustic blockwhich comprises taking a mass of individual small particles of rock sponge, dampening the mass of particles, mixing the dampened particles with a dry powdered binding material, whereby the moisture from the particles will be taken up by said binding inaterial, and then removing the excess moisure.

4. An acoustic material comprising a pressed block formed of small particles of rock sponge, said particles being mixed with a bonding material, the'sharp edges of the particles having been broken down by the pressure so that the particles are brought into intimate contacting relation.

5. An acoustic material comprising a block formed of small particles of rock sponge bonded together, said particles being of assorted sizes whereby the interstices between the larger particles Willbe substantially filled by particles of the smaller sizes.

6. An acoustic block formed of small particles of rock sponge bonded together, said particles being of irregular shape and the finished block being substantially free of interstices between the particles 7. A sound absorbing material comprising an assembled mass of particles of rock sponge, the individual particles being of ir- I regular sizes and shapes, whereby the smaller particles will tend to fill up the interstices between the larger particles, the individual particles having such a multiplicity of air containing cells therein as to be at least temporarily floatable in water, and said particles being held together by a bonding agent.

8. A sound absorbing material comprising a mass of particles of rock sponge bonded together by a suitable bonding agent, the indiyidual particles having such a multiplicity of air containing cells and intercommunicating passageways therein as to be at least temporarily floatable in water.

9. A sound absorbing material comprising an assembled mass of particles of rock sponge, the individual particles having such a multiplicity of air containin cells as to be at least temporarily floatable 1n water,.and

means for holding said particles in assemfi bled relation.

10. A sound absorbing material comprising a mass of particles of rock sponge bonded together by asuitable bonding agent, the individual particles having such a multiplic- 10 ity of air containing cells and intercommunieating passageways therein as to be at least temporarily floatable in water, said material having a reinforcing backing material permanently secured thereto.

11. A sound absorbing material comprising a layer of particles of rock sponge mixed together with a fibrous material and the mixture bonded together by a suitable bonding agent, the individual particles of rock sponge having such a multiplicity of air containing cells as to be at least temporarily floatable in water.

12. A sound absorbin material comprising a layer of particles 0 rock sponge mixed together with a fibrous material and the mixture bonded together by a suitable bonding agent, the individual particles of rock sponge having such a multiplicity of air containing cells as to be at least temporarily floatable in water, and a sheet of reinforcing material secured to the back of said layer of particles.

In Witness whereof, I hereunto subscribe my name this 12th day of December, 1930.

' WILLIAM T. DEAN. 

